Lean-burn engines, such as those used by diesel-powered and natural gas-powered vehicles, equipment, and generators combust at high air-to-fuel ratios in comparison with rich-burn engines. By nature, lean-burn engines use less fuel while producing equivalent power of similar-sized rich-burn engines. However, lean-burn engines have increased NOX and NH3 emissions generally due to the slow burn rates associated with the lean mixtures of fuel (i.e., excess air introduced with the fuel). Exhaust aftertreatment systems are typically used to reduce NOX and NH3 emissions from the exhaust of the lean-burn engines.
Generally described, the aftertreatment systems may include a selective catalyst reduction system. The selective catalyst reduction system adds a reductant, typically ammonia or urea, to the combustion gas stream before passing the stream through a catalyst bed so as to absorb selectively the nitrogen oxides and the reducing agent. The absorbed components undergo a chemical reaction on the catalyst surface and the reaction products are desorbed. Specifically, the reactant reacts with the nitrogen oxides in the combustion gas stream to form water and nitrogen. The aftertreatment systems also may include a secondary oxidation catalyst. The secondary catalyst may include of a substrate made up of numerous small channels coated with a porous layer containing catalysts (e.g., platinum, palladium). As the exhaust gases traverse the channels, certain chemicals (e.g., carbon monoxide (CO), hydrocarbon (HC)) may react with oxygen to form carbon dioxide (CO2) and water vapor. Other types of catalysts and other types of reductants may be used herein.
Typical aftertreatment system, particularly for lean burn engines, may have certain limitations because any NH3 that slips from the selective catalyst reduction system may be oxidized back into NOx, defeating the purpose of the NOx aftertreatment. Careful metering and distribution of the reductant to the selective catalyst reduction system may be require to convert and remove a sufficient level of the nitrogen oxides and the like. Moreover, NOx, NH3, CO, and HCHO emissions may change significantly across the aftertreatment system due to coupled reactions occurring in the secondary catalyst, especially as the catalysts age. Proper monitoring of the reductant used in the aftertreatment system thus is required for both efficient operation and emissions compliant operation.